He is best known for having synthesized acetylsalicylic acid (ASA) on August 10, 1897, supposedly for the first time in a stable form usable for medical applications. Bayer marketed this substance as Aspirin. However, this has been disputed. In 1949, Arthur Eichengrün published a paper in which he claimed to have planned and directed the synthesis of Aspirin along with the synthesis of several related compounds. He also claimed to be responsible for Aspirin's initial surreptitious clinical testing. Finally, he claimed that Hoffmann's role was restricted to the initial lab synthesis using his (Eichengrün's) process and nothing more.
The Eichengrün version was ignored by historians and chemists until 1999, when Walter Sneader of the Department of Pharmaceutical Sciences at the University of Strathclyde in Glasgow re-examined the case and came to the conclusion that indeed Eichengrün's account was convincing and correct and that Eichengrün deserved credit for the invention of Aspirin. Bayer promptly denied this theory in a press release, claiming that the invention of Aspirin was due to Hoffmann.
As of 2004, the controversy is still open: while Sneader's version has been widely reported, there are no independent second sources supporting either version.
Both substances had been synthesized earlier, but not in forms that could be used for medication. ASA had first been synthesized by Frenchman Charles Frédéric Gerhardt in 1853, and diacetylmorphine (that is, heroin) by C.R. Alder Wright, a British chemist in 1873.
Following the synthesis of aspirin, he changed to the pharmaceutical marketing department, where he stayed until his retirement in 1928. In 2002, he was inducted into the US National Inventors Hall of Fame.